Investigation on the mechanical behaviour of single-lap composite joints with countersunk bolts

Abstract

In this thesis, the mechanical behaviour of 2-bolt single-lap CFRP joints with countersunk bolts is investigated, both numerically and experimentally. A detailed 3D non-linear FE model of composite bolted joints has been developed. The model replicates with good agreement the experimental tensile tests up to the point where bearing damage occurs and reproduces the joint behaviour correctly. Five stages are identified in the joint behaviour. The evolution of contact during the test is studied showing a correlation with the joint stiffness. Parametric studies investigate the influence of bolt clamping force, coefficient of friction and bolt-hole clearance on the joint behaviour. Using the developed FE model, the distribution of the six stress components around the holes of the composite joints is studied together with the effects of head height, shank-hole clearance and position of bolts. The stress along the fibres is identified as the critical stress component and the compressive fibre failure of the 0° oriented plies as the start of the bearing damage. The 0° oriented plies in the cylindrical part of the hole are found to be the plies carrying the bearing load. Increasing clearance reduces the extent of the bolt shank-hole contact and leads to higher stresses and lower joint stiffness. The 45° and -45° oriented plies are found to have a key role in the joint bearing strength and shear-out failure. Fatigue tests are run with the introduction of a novel method to monitor the loss of clamping force and detect crack initiation in the fasteners during the test itself, using strain gauges and a real-time algorithm. The clamping force is found to remain constant until crack initiation and progressively drop until final failure. Load transfer and interaction between the bolts is observed during the fast but stable fatigue crack propagation. The fatigue tests are conducted on joints with differing plate thicknesses and countersunk head geometries to assess the influence of these on the number of cycles to crack initiation and to final failure. A link between fatigue and static test results is highlighted and, in addition, a fatigue failure mechanism of the joint is proposed.